Saamite, Ba□TiNbNa3Ti(Si2O7)2O2(OH)2(H2O)2, a Group-III Ti-disilicate mineral from the Khibiny alkaline massif, Kola Peninsula, Russia : description and crystal structure

Saamite, Ba□TiNbNa3Ti(Si2O7)2O2(OH)2(H2O)2, is a Group-III TS-block mineral from the Kirovskii mine, Mount Kukisvumchorr, Khibiny alkaline massif, Kola Peninsula, Russia. The mineral occurs as transparent platy crystals 2-10 μm thick and up to 180 μm across. It is colorless to very pale tan, with a white streak and a vitreous luster. The mineral formed in a pegmatite as a result of hydrothermal activity. Associated minerals are natrolite, barytolamprophyllite, kazanskyite, nechelyustovite, hydroxylapatite, belovite-(La), belovite-(Ce), gaidonnayite, nenadkevichite, epididymite, apophyllite-(KF), and sphalerite. Saamite has perfect cleavage on {001}, uneven fracture and a Mohs hardness ca. 3. Its calculated density is 3.243 g/cm3. Saamite is biaxial positive with α 1.760, β 1.770, γ 1.795 (λ 589 nm), 2Vmeas. = 69(2)° , 2Vcalc. = 65° , with medium dispersion, r > v. It is nonpleochroic. Saamite is triclinic, space group P1¯, a 5.437(2), b 7.141(3), c 21.69(1) Å, α 92.97(1), β 96.07(1), γ 90.01(1)° , V 836.3(11) Å3. The strongest lines in the X-ray powder-diffraction pattern [d(Å)(I)(hkl)] are: 21.539(100)(001), 2.790(15)(122), 2.692(14) (008), 3.077(13)(007), 7.180(11)(003), 2.865(11)(12¯2), 1.785(9)(11¯4), 2.887(9)(1¯22, 01¯7, 115), and 1.785(9)(04¯1, 13¯7, 040,2¯2¯8, 230, 231¯). Chemical analysis by electron microprobe gave Nb2O5 12.24, TiO2 20.37, SiO2 29.07, Al2O3 0.08, FeO 0.32, MnO 5.87, MgO 0.04, BaO 11.31, SrO 2.51, CaO 1.76, K2O 0.77, Na2O 8.39, H2O 5.77, F 1.71, O = F -0.72, sum 99.49 wt.%; H2O was determined from structure refinement and its presence was confirmed by IR spectroscopy. The empirical formula based on 20 (O + F) atoms pfu is (Ba0.61Sr0.20K0.13□0.06)∑1(□0.74Ca0.26) ∑1(Na2.22Mn0.55Fe2+0.04□0.19)∑3 (Ti2.09Nb0.76Mn0.13Mg0.01Al0.01)∑3Si3.97O19.26H5.26F0.74, Z = 2. The simplified formula is as follows: Ba(□,Ca)Ti(Nb,Ti)(Na, Mn)3(Ti,Nb)(Si2O7)2O2(OH,F)2(H2O)2. The IR spectrum of saamite contains the following bands: ∼1605, 1645, ∼1747 and ∼3420 cm-1. The crystal structure was solved by direct methods and refined to an R1 index of 9.92%. In the crystal structure of saamite, the main structural unit is the TS block, which consists of HOH sheets (H-heteropolyhedral, O-octahedral). The TS block exhibits linkage and stereochemistry typical for Group III [Ti (+ Nb + Mg) = 3 apfu] of TS-block minerals. The O sheet is composed of Na- and Ti-dominant octahedra and has ideal composition Na3Ti apfu. The TS block has two different H sheets where Si2O7 groups link to [5]-coordinated Ti and [6]-coordinated Nb polyhedra, respectively. There are two peripheral sites, [10]-coordinated AP(1) and [8]-coordinated AP(2), occupied mainly by Ba (less Sr and K) at 94% and Ca at 26%, respectively. In the crystal structure of saamite, adjacent TS blocks connect in two different ways: (1) via hydrogen bonds between H2O- H2O groups and H2O-O atoms of adjacent TS blocks; (2) via a layer of Ba atoms that constitute the I block. The TS block, I block and types of self-linkage of TS blocks are topologically identical to those in the nechelyustovite structure. The mineral is named after the Saami (Caamn in Cyrillic) indigenous people who inhabit parts of the Kola Peninsula of Russia, far northern Norway, Sweden, and Finland.